Structures of Uncharacterised Polymorphs of Gallium Oxide from Total Neutron Diffraction

Abstract A structural investigation is reported of polymorphs of Ga 2 O 3 that, despite much interest in their properties, have hitherto remained uncharacterised due to structural disorder. The most crystalline sample yet reported of γ‐Ga 2 O 3 was prepared by solvothermal oxidation of gallium metal in ethanolamine. Structure refinement using the Rietveld method reveals γ‐Ga 2 O 3 has a defect Fd $\bar 3$ m spinel structure, while pair distribution function analysis shows that the short‐range structure is better modelled with local F $\bar 4$ 3 m symmetry. In further solvothermal oxidation reactions a novel gallium oxyhydroxide, Ga 5 O 7 (OH), is formed, the thermal decomposition of which reveals a new, transient gallium oxide polymorph, κ‐Ga 2 O 3 , before transformation into β‐Ga 2 O 3. In contrast, the thermal decomposition of Ga(NO 3 ) 3 ⋅9 H 2 O first forms ε‐Ga 2 O 3 and then β‐Ga 2 O 3 . Examination of in situ thermodiffraction data shows that ε‐Ga 2 O 3 is always contaminated with β‐Ga 2 O 3 and with this knowledge a model for its structure was deduced and refined—space group P 6 3 mc with a ratio of tetrahedral/octahedral gallium of 2.2:1 in close‐packed oxide layers. Importantly, thermodiffraction provides no evidence for the existence of the speculated bixbyite structured δ‐Ga 2 O 3 ; at the early stages of thermal decomposition of Ga(NO 3 ) 3 ⋅9 H 2 O the first distinct phase formed is merely small particles of ε‐Ga 2 O 3 .